Antennas having moderately high gain and a broad bandwidth are particularly useful in electromagnetic susceptibility testing of electronic equipment region because they obviate the movement of the device under test from one test location to another in order to expose the device to radiation at all of the frequencies of interest.
The so-called “log-periodic dipole” antenna is an ideal type of antenna for susceptibility testing over the VHF and UHF ranges not only in the radiating far field region, but also in the radiating near field region, i.e. the region within a distance less than approximately from the antenna. A basic log-periodic dipole antenna λ/2π from the antenna. A basic log-periodic dipole antenna is described in U.S. Pat. No. 3,210,767, dated Oct. 5, 1965. The antenna is a coplanar dipole array consisting of array of dipoles of progressively increasing length and spacing in side-by-side relationship, the dipoles being fed by a common feeder which extends from a forward end to a rearward end and alternates in phase between successive dipoles. The ratio of the lengths of the successive dipoles is given by             L              (                  n          +          1                )                    L      n        =  τwhere                Ln is the length of any intermediate dipole in the array;        L(n+1) is the length of the adjacent shorter dipole; and        τ is a constant having a value less than 1, preferably from 0.8 to 0.95.The ratio of the spacings between dipoles is given by             Δ      ⁢                           ⁢              S                  (                      n            +            1                    )                            Δ      ⁢                           ⁢              S        n              =  τwhere        ΔSn is the spacing between the dipole having the length Ln and the adjacent larger dipole; and        ΔS(n+1) is the spacing between the dipole having the length Ln and the adjacent smaller dipole. Thus, the lengths of the dipoles, and their spacings, progress logarithmically.        
The log-periodic dipole antenna typically has a very broad frequency range, a power gain in the range from 6 to 8 dBi (deciBels over isotropic), and a relatively constant input impedance, allowing it to be utilized for testing over the entire spectrum of interest in most cases. Thus, it is usually unnecessary to move the device under test from one test location to another, or to change antennas for different frequency ranges.
However, even when a conventional log-periodic dipole antenna is used for susceptibility testing, it is frequently necessary to adjust the position of the device under test relative to the antenna, or to adjust the r.f. power fed to the antenna, in order to expose the device to the appropriate field, strength, especially at low frequencies. The problem at low frequencies is that the active region (or phase center), that is, the point on the antenna at which it appears that the field is emanating, moves rearward with decreasing frequency because the longer dipoles, which are more remote from the device under test, come into play.